The superblock could even behave as a genesis block of sorts in that full clientś and miners might not be required to download previous blocks.
I think they still would have to download previous blocks because a block only contains 10 minutes worth of transactions. It doesn't know all the inputs and outputs of what came before it.
A checkpointing sblock would contain not only 10 minutes worth of transactions but a snapshot of the entire chain. You can look at it as making a new pre-mined coin where the pre-mine goes to all addresses which had coin on the old chain. One could even play some more tricks like adjusting the sblock difficulty in proportion to the amount of data collecting in the chain to target a certain portability.
